Teeth play a critically important role in our lives. Loss of function reduces the ability to eat a balanced diet which results in negative consequences for systemic health. Loss of aesthetics can negatively impact social function. Both function and aesthetics can be restored with dental crowns and bridges. Ceramics are attractive dental restoration materials because of their aesthetics, inertness, and biocompatibility. However, ceramics are brittle and subject to premature failure, especially after repeated contact including slide-liftoff masticatory loading in a moist environment (Kim et al. (2007)Journal of Dental Research 86(11): 1046-1050; Lawn et al. (2001) The Journal of Prosthetic Dentistry 86(5): 495-510; Lawn et al. (2001) J Prosthet Dent 86(5): 495-510; Zhang et al. (in press) “Fatigue Damage in Ceramic Coatings from Cyclic Contact Loading with Tangential Component.” Journal of the American Ceramic Society) Fracture rates of ceramic restorations may seem low at 3-4% per year (Fradeani et al. (1997) Int. J. Prothodont. 10: 241-7; Malament et al. (1999) J. Prosthet. Dent. 81: 23-32; Sjogren et al. (1999) Int. J. Prosthodont. 12: 122-8; Sailer et al. (2006) Quintessence International 37(9): 685-693; Sailer et al. (2007) Clin. Oral Impl. Res. 18(3): 86-96; Pjetursson et al. (2007) Clin. Oral Impl. Res. 18(3): 73-85). However, failure can cause significant patient discomfort and loss of productive lifestyle. The vulnerability of dental ceramic restorations is exacerbated by damage, fatigue loading, and moisture.
According to a survey conducted by American Dental Association, more than 45 million new dental crowns, of which over 37 million were porcelain (ceramic) based, were provided by dentists in 1999 (ADA (2002). “The 1999 Survey of Dental Services Rendered.”). As the population ages, the number will increase. Despite continuous efforts to improve the strength of dental ceramics, all-ceramic dental crowns continue to fail at a rate of approximately 3-4% each year (Burke et al. (2002) J Adhes Dent 4(1): 7-22). The highest fracture rates are on posterior crowns and bridges where stresses are greatest. Dental crowns generate over $2 billion each year in revenues with 20% of the units being all-ceramic (Nobel Biocare 2004). Dental ceramics that best mimic the optical properties of enamel and dentin are predominantly glassy materials principally feldspar (a group of minerals having main constituents of silica and alumina) (Kelly (1997) Annual Reviews of Materials Science 27: 443-68; Kelly (2004) Dent. Clin. N. Am. 48: 513-30). The original dental porcelain contained high feldspathic glass content and was extremely brittle and weak (S (strength) approximately ˜60 PMa) (McLean, J. W. (1979) The Science and Art of Dental Ceramics. Chicago, Quintessence Publishing Co. Inc.; Binns, D. (1983) The Chemical and Physical Properties of Dental Porcelain. Chicago, Quintessence Publishing Co. Inc.). Therefore, despite the aesthetic advantage, the early porcelain crowns were not widely used in dentistry (Van, N. R. (2002). “An Introduction to Dental Materials.” 231-46).
Today, about 0.1% of the population in North America and Europe need a total hip replacement (THR), and over 300,000 total knee replacement surgeries are performed each year in the United States. (Willmann G. Ceramics for Total Hip Replacement—What a Surgeon Should Know. 1998, 21(2):173-7:NIH, J Bone Joint Surg Am 2004;86-A(6):1328-1335) The overall orthopedic implants, including replacement of knees, hips, fingers, and spinal processes, are estimated to have a world-wide market exceeding $4.3 billion annually. (Davis J R. Handbook of materials for medical devices materials. Park, Ohio; 2003). On the other hand, dental crowns and bridges generate over $2 billion each year in revenues with 20% of the units being all-ceramic. (Dental Market Overview Nobel Biocare; 2004) The demand for aesthetics will likely drive the number of all-ceramic prostheses even higher. Despite a continuous effort in improving the properties of medical and dental ceramics, ceramic prostheses are still vulnerable to wear and surface damage, especially in repeated loading in wet environments. Wear characteristic and catastrophic failure of ceramic-on-ceramic articulations continue to be a concern. (Barrack et al., Clinical Orthopaedics and Related Research 2004;429:73-79) All-ceramic dental crowns, including porcelain-veneered alumina and zirconia, continue to fail at a rate of approximately 1-3% each year. (Burke et al., J Adhes Dent 2002;4(1):7-22; Sailer et al., Clin. Oral Impl. Res. 2007;18(3):86-96)
Dental ceramics have become increasingly popular as restorative materials due to improvements in strength. Several methods have been developed to improve the strength of dental ceramics including adding uniformly disperse appropriate filler particles throughout a glass matrix, referred to as “dispersion strengthening” (McLean et al. (1965) Br. Dent. J. 119: 251-67). The first fillers used in dental ceramics were leucite particles (Denry (1996) Crit. Rev. Oral. Biol. Med. 7: 134-43). Commercial dental ceramics containing leucite as a dispersion strengthening fillers include IPS Empress (S approximately 120 PMa) (Ivoclar-Vivadent, Schaan, Liechtenstein) and Finesse All-ceramic (S approximately 125 MPa) (Dentsply Prosthetics, York, Pa.). Particle strengthening can also be achieved by heat-treating the glass to facilitate the precipitation and subsequent growth of crystallites within the glass, termed “ceraming”. Dental ceramics produced using the ceraming process are called glass-ceramics. Several commercial products such as Dicor (S approximately 160 MPa) (Dentsply), IPS Empress II (S approximately 350 MPa) (Ivoclar-Vivadent) and, more recently, IPS e.max Press (S approximately 525 MPa) (Ivoclar-Vivadent) are examples. The leucite-strengthened porcelains and the glass-ceramics are translucent, so single layer (monolithic) crowns can be made from these materials. However, only moderate strength increases can be achieved via the particle strengthening techniques. Therefore, monolithic ceramic crowns experience high failure rates range from 4-6% for Dicor molar crowns (Malament et al. (1999) J. Prosthet. Dent. 81: 23-32) and 3-4% per year for IPS Empress crowns (Fradeani et al. (1997) Int. J. Prothodont. 10: 241-7; Sjogren et al. (1999). Int. J. Prosthodont. 12: 122-8). Note: comprehensive clinical reports on the new IPS e.max Press crowns are not available at this stage.
The current approach to the fracture problem of monolithic crowns is a layer-structure with aesthetic but weak porcelain veneers fused onto strong but opaque ceramic cores. This involves an increase in crystalline content (from approximately 40 vol. % to 99.9 vol. %) accompanied by a reduction in glass content. The first successful strengthened core ceramic was made of feldspathic glass filled with approximately 40 vol % alumina particles (McLean et al. (1965). Br. Dent. J. 119: 251-67). The alumina fillers increased the flexural strength of the ceramic to approximately 120 MPa with a trade off in translucency; hence veneering was required. Using McLean's approach, in 1983 Coors Biomedical (Golden, Colo.) developed Cerestore all-ceramic crowns with a ceramic core containing approximately 60 vol. % of alumina (Sozio et al. (1983). J. Prosthet. Dent. 69: 1982-5). However, following problems with fractured crowns the manufacturer withdrew the system. A similar product from the same era, the Hi-Ceram crown (Vita, Bad Säckingen, Germany) with its core material containing about the same amount of alumina as the Cerestore core, also failed to meet the satisfactory for posterior restorations (Bieniek et al. (1994). Schweitz Monatsschr Zahnmed 104: 284-9). The Hi-Ceram crown was replaced by In-ceram crown (Vita) in 1990. The In-ceram crown had a core that was fabricated by lightly sintering an alumina powder compact and then infiltrating the still porous alumina matrix with a low viscosity glass. The final core material contained approximately 70 vol. % of alumina and had a flexural strength of approximately 450 MPa (Probster (1992) Int J Prosthodont 5(5): 409-14). In 1993, Procera (Nobel Biocare, Göteborg, Sweden) presented the all-ceramic crown concept (Anderson et al. (1993). Acta Odontol Scand 51: 59-64), where the fully dense core material contained 99.9 vol % alumina and displayed a flexural strength of 675 MPa. Several years later, even stronger Y-TZP ceramic was introduced to dentistry as a core material with a flexural strength over 1200 MPa.
Unfortunately, no current materials, including stronger monolithic ceramics (orthopedic and dental prostheses) or strong cores to support weak, but aesthetic porcelain veneers (dental prostheses) can effectively suppress both contact and flexural damages. In addition, veneered strong ceramic dental prostheses have a dense, high purity crystalline structure at the cementation internal surface that cannot be readily adhesively bonded to tooth dentin as support. Surface roughening treatment such as particle abrasion is commonly used to enhance the ceramic-luting agent bond. However, particle abrasion also introduces surface flaws or microcracks that can cause deterioration in the long-term flexural strength of ceramic prostheses. (Zhang et al. (2004) Journal of Biomedical materials research 71B(2): 381-6; Zhang et al. (2005) Journal of Biomedical materials research 72B: 388-92; Zhang et al. (2006) The International Journal of Prosthodontics 19(5): 442-8).
Recent advances in theoretical and experimental work have shown that functionally graded materials, referred to as FGMs, may provide unprecedented resistance to contact damage (Suresh et al. (2003) U.S. Pat. No. 6,641,893; Suresh et al. (1997) Acta Materialia 45(4): 1307-21; Jitcharoen et al. (1998) Journal of the American Ceramic Society 81(9): 2301-8; Suresh et al. (1999) Acta Materialia 47(14): 3915-3926). Such damage resistance cannot be realized with conventional homogeneous materials. FGMs are made of two materials that are combined so that the surface of the FGM is composed entirely of material A, and the interior is composed entirely of material B. Additionally, there is a continuous change in the relative proportions of the two materials from the surface to interior. One known FGM is a thick ceramic block, alumina or silicon nitride, infiltrated with a low elastic modulus aluminosilicate glass or oxynitride glass (SiAlYON), respectively, on one surface to produce a graded glass/ceramic (G/C) structure that suppresses contact damage at the top, occlusal surface (Jitcharoen et al. (1998) Journal of the American Ceramic Society 81(9): 2301-8). However, upon infiltration of dense ceramics, the glass penetrates the grain boundaries and grain boundary triple junctions, and as a result, the ceramic grains gradually separate. This leads to an increase in volume at the surface of graded structure and is accompanied by warpage or bending of the specimens where the glass-impregnated surface is convex.
Aluminium oxide is an amphoteric oxide of aluminium with the chemical formula Al2O3. It is also commonly referred to as alumina in the mining, ceramic and materials science communities. It is produced by the Bayer process from bauxite; its most significant use is in the production of aluminium metal. Being very hard, it is used as an abrasive. Having a high melting point, it is used as a refractory material. Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity. In its most commonly occurring crystalline form, called corundum or α-aluminum oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools. Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of alumina quickly forms on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The alumina generated by anodising is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.